1. Field of Invention
The invention relates to a thermal fixing device and an image forming device having a thermal fixing device. Specifically, the invention relates to a thermal fixing device that can promptly and properly prevent itself from overheating when the thermal fixing device is excessively heated due to a malfunction of a control circuit or other defects, and the invention also relates to an image forming device having such a thermal fixing device.
2. Description of Related Art
An image forming device, such as a laser printer, is typically provided with a thermal fixing device having a heat roller and a pressure roller in order to fix transferred toner onto a sheet. While the sheet with a transferred toner image is passing between the heat roller and the pressure roller, the thermal fixing device thermally melts the toner and fixes it onto the sheet as a fixed toner image.
A heater lamp is typically build in a heat roller of such a thermal fixing device, as recited in Japanese Laid-Open Publication No. 5-188824. Additionally, a temperature sensor, such as a thermister, is disposed around the heat roller. The temperature of the heat roller is sensed by the temperature sensor. The heat roller is kept at a generally constant temperature by a control circuit that turns on/off the power source of the heater lamp based on the temperature sensed by the temperature sensor.
When temperature control of the heat roller is disabled due to a malfunction of the control circuit or other defects, a thermal runaway may occur. When the power source of the heater lamp cannot be turned off, the temperature of the heat roller keeps rising. To prevent overheating caused by such a thermal runaway, a current circuit breaker, such as a temperature fuse and a thermostat, is provided around the heat roller. The electric circuit breaker forcibly interrupts power supply to the heater lamp upon sensing a temperature beyond a specified temperature. As disclosed in Japanese Laid-Open Publication No. 5-188824, the temperature sensor and the temperature fuse are disposed at one end of the heat roller.
Usually, as shown in FIG. 6, temperature distribution curves of the heater at the ends of the heat roller differ greatly depending on whether the heating is started or stabilized. In addition, the temperature of the heat roller differs greatly between its center and its ends. For these above-mentioned reasons, accurate sensing of the temperature of the heat roller hardly attains to the accuracy of the thermal fixing device recited in Japanese Laid-Open Publication No. 5-188824. If the temperature of the heat roller cannot be accurately sensed, it would be difficult to keep the temperature required for thermal fixing at a generally constant temperature and to activate the temperature fuse accurately.
In view of the foregoing, the invention provides a thermal fixing device that can accurately sense the temperature of a heat roller and activate a thermal cutoff device promptly and properly when the heat roller is excessively heated. The invention also provides an image forming device having such a thermal fixing device.
According to one aspect of the invention, a thermal fixing device includes a roller, a heater that heats the roller, a temperature sensor provided around the roller to sense a temperature of the roller, and a thermal cutoff device provided around the roller to interrupt power supply to the heater. The heater has a temperature distribution that is symmetrical in an axial direction of the roller with respect to a temperature distribution center, and the temperature sensor and the thermal cutoff device are disposed facing a surface of the roller in alignment with the symmetrical positions spaced by a predetermined distance, in the axial direction of the roller, from the temperature distribution center.
With this configuration, the thermal cutoff device can sense substantially the same temperature as that sensed by the temperature sensor. Accordingly, the upper limit temperature at which the thermal cutoff device functions can be set closer to the upper limit of the control temperature and, as a result, the thermal cutoff device can promptly and properly prevent the roller from overheating.